The purpose of the invention is to improve stripmap synthetic aperture radar/sonar (SAR/SAS) and inverse synthetic aperture radar/sonar/ultrasound (ISAR/ISAS) via better clutter rejection, velocity parameter estimation (range rate and azimuth rate), and azimuth position estimation. The invention also is applicable to target movement in three dimensions relative to a radar/sonar/ultrasound array. In this case, the invention improves estimates of range rate, azimuth rate, elevation rate, azimuth position, and elevation position. Applications are to maritime and ground surveillance SAR containing moving objects, missile defense radar (ISAR), sonar mine hunting (SAS), and noninvasive Doppler ultrasound (ISAS) for fluid velocity measurement in two dimensions (parallel and perpendicular to the vessel).
In synthetic aperture processors, echoes are coherently pulse compressed (replica correlated or matched filtered) on reception and used to form in-phase and quadrature components. These components specify the magnitude and phase of a complex-valued echo range sample. A sequence of such components from multiple signal-echo pairs comprise a phase history corresponding to the range variation (measured in wavelengths) associated with relative motion between the radar/sonar platform and each point target.
If point targets at different azimuth locations do not move except for relative platform motion, then their phase histories are azimuth-displaced versions of a predictable phase history, and they can be separated by azimuth compression if the phase history bandwidth B is sufficiently large (C. E. Cook and M. Bernfeld, Radar Signals, Academic Press, New York, 1967). If various point targets have different range rates, however, they may not be separable even if B is large. This lack of resolution can occur if range rate and azimuth displacement can compensate for one another. For example, range rate causes a displacement in frequency. If a phase history lies on a long tilted line in the time-frequency plane, a frequency displacement can be compensated by a time shift. A phase history time shift is equivalent to an azimuth shift. A tilted line in the time-frequency plane corresponds to linear frequency modulation (linear FM) and quadratic phase modulation. Quadratically phase modulated (linear FM) phase histories experience degraded range-rate/azimuth resolution with ambiguous receiver outputs, such that objects with nonzero range rate appear at the wrong azimuth.
Azimuth compression often utilizes frequency domain matched filtering to correlate the data phase history with reference phase histories. An unknown range rate causes a Doppler shift, which can be hypothesized by a frequency domain shift of a predicted (reference) phase history. Predicted phase histories are correlated with the data phase history for estimation/detection. Range rate estimation accuracy and resolution capability is proportional to the duration T of the phase histories in the absence of ambiguities and error coupling, such as those that occur with linear FM (Cook and Bernfeld, op. cit.).
Azimuth rate in the direction of assumed relative platform motion increases the rate at which the beam pattern sweeps across a target, and causes time compression of phase histories; azimuth rate in the opposite direction causes time dilation. Time scaling (compression/dilation) can be included as an additional parameter hypothesis in the azimuth compression process. The effect of nonzero range rate on a wideband radar/sonar waveform also is represented by time scaling. Estimation accuracy of compression/dilation increases with waveform time-bandwidth product (R. A. Altes and E. L. Titlebaum, “Bat signals as optimally Doppler tolerant waveforms,” J. Acous. Soc. Am. Vol. 48, 1970, pp. 1014-1020). Azimuth rate estimation accuracy thus is proportional to the time-bandwidth product (TB) of the phase histories, unless ambiguity effects limit estimation and resolution capability.
For the smooth, low time-bandwidth product beam patterns that comprise prior art, the phase history caused by relative target/platform motion in broadside stripmap SAR is closely approximated by a quadratic phase function, corresponding to linear FM. The azimuth compression process is then subject to the well known range-Doppler error coupling phenomenon for linear FM, such that the receiver response to ambiguous pairs of erroneous azimuth displacements and range rates is nearly as large as the receiver response to the correctly hypothesized azimuth and range rate. This error coupling is manifested as a ridge in the phase history azimuth/range-rate ambiguity function, and it severely degrades estimation/detection performance in a cluttered environment relative to a receiver with an ideal (thumbtack) ambiguity function. Linear FM also is relatively insensitive to azimuth rate (compression/dilation), compared to other waveforms or phase histories with the same TB product (R. A. Altes, “Optimum waveforms for sonar velocity discrimination,” Proc. of the IEEE vol. 39, 1971, pp. 1615-1617).
In the absence of error coupling, azimuth resolution improves as SAR/SAS beam width is increased, since phase history bandwidth is increased. Increased beam width also increases the phase history time-bandwidth product, resulting in improved azimuth rate resolution. Increased beam width and phase history duration, however, cause extension of the tilted line representation of linear FM in the time-frequency plane, with consequent extension and flattening of the linear FM ambiguity function ridge line. These effects signify an increase in the effect of unknown range rate on azimuth estimation error. A trade-off thus occurs, such that beam widening improves azimuth rate estimation but degrades azimuth and range rate estimation because of linear FM error coupling. Target detection in Doppler-distributed clutter also tends to be degraded when the FM ridge line is extended via beam widening. Synthetic aperture processors that use beams with no phase modulation are geometrically constrained to operate with quadratic phase histories (linear FM) for azimuth compression, despite the drawbacks and trade-offs associated with such modulation. This constrained operation constitutes the prior art.
The invention is to replace a conventional beam pattern (without nonlinear phase modulation) with a beam pattern that has nonlinear phase modulation or phase coding. Such a beam pattern is obtained by phase modulating the aperture shading function, which broadens the beam as well as imparting the desired phase modulation to the beam pattern. As the beam is swept past a point target in a SAR/SAS application (or a point target moves through the beam in ISAR/ISAS), beam coding/modulation adds beam-induced phase variation to the quadratic phase that is associated with range variation. The added beam-induced phase is controlled by the system designer rather than by geometry.
An appropriate beam phase modulation function removes the constraints that have been imposed by linear FM phase histories. In a broadside stripmap SAR, appropriate beam phase modulation dramatically reduces range-rate/azimuth coupling error, greatly improves resolution and detection/estimation performance in clutter, significantly reduces estimation errors for joint estimation of azimuth, range rate, and azimuth rate, and eliminates the trade-offs associated with conventional SAR beam width variation when phase histories have quadratic phase variation. For three dimensional operation, similar advantages apply to estimates of azimuth, elevation, azimuth rate, elevation rate, and range rate, provided that a two dimensional beam pattern is coded/modulated in azimuth and elevation so as to reduce ambiguities.
Applications include discrimination of small objects from sea clutter in maritime stripmap SAR and discrimination of moving vehicles from stationary roadside objects in ground mapping stripmap SAR. Comparison of SAR ground maps obtained at two different times can be used to identify new roadside objects. The invention enables accurate, noninvasive ISAS measurement of range, azimuth, range rate, and azimuth rate, thus creating informative representations of fluid flow parallel and orthogonal to a vessel's length, as a function of distance from the vessel wall.